22 years give or take, started with SLS, can't remember which version probably older than this one ftp://ftp.ibiblio.org/pub/hist... as I can vaguely remember kernel 0.97 but SLS 1.03 has kernel 0.98pl.

kvm (with a little patch) supports it, running it right now with 5 guests and have 53K pages which are shared.
# cat/sys/kernel/mm/ksm/pages_sharing
53714
That's ~200MB for about 1,5GB memory used on the host.
Now I can't figured out how many times those pages are shared, so I can't calculated the actual memory saved (it's between 200MB and 4x200MB).